The present invention relates to a receiving method and apparatus which performs a visited cell search and a neighbor cell search in radio communications using a CDMA radio access system.
With a conventional receiving apparatus in the CDMA radio communication system, a mobile station uses what is called a dedicated searcher whereby, at turn-on or during the standby state, a decision is made about the cell where the mobile station ought to stay or a search is conducted for surrounding cells or a multi-path that the mobile station receives.
In FIG. 1A there is shown in block form an example of the configuration of the receiver of the mobile station for the conventional CDMA radio communication. A signal received by an antenna 11 is demodulated by a radio part 12 into a base band signal, which is converted by an A-D converter 13 to a digital signal. Code generators 14.sub.1 to 14.sub.4 are each given spreading code information and frame timing detected by a multi-path searcher 15, and hence they generate codes synchronized with spreading codes used at the sending side. In correlators 16.sub.1 to 16.sub.4 the base band signal from the A-D converter 13 is despread through multiplication by the spreading codes from the code generators 14.sub.1 to 14.sub.4, by which the original signal can be detected. Thereafter, these despread signals are detected by detectors 17.sub.1 to 17.sub.4 and the detected outputs are combined by a RAKE combiner 18, whose combined output is fed to a signal processing part 19. Since signals from different transmission or propagation paths (a multi-path), contained in the received signal, can be separated by the despread processing in the correlators 14.sub.1 to 14.sub.4, this processing is a RAKE reception that has a diversity effect.
The mobile station receiver of the prior art configuration is provided with receiving fingers Fn.sub.i each composed of the code generator 14.sub.i, the correlator 16.sub.i and the detector 17.sub.i for despreading and detecting the received signal and the multi-path searcher 15 provided independently of the receiving finger Fn.sub.i to search for the multi-path in the receiving channel. A brief explanation will be given of the start-up operation of the mobile station at its turn-on, that is, an operation from a decision about its visited cell to the start of signal reception. When power is turned on, the mobile station accepts the base band signal from the A-D converter 13 by the multi-path searcher 15 and searches for the cell where the mobile station ought to stay. The multi-path searcher 15 has such a configuration as shown in FIG. 1B. A code number designator 23 reads out of a memory 24 a candidate for the spreading code of the base station and indicates to a code generator 25 a spreading code to be generated. The code generator 25 generates the designated spreading code, which is fed to a multiplier 26 for multiplication by the input signal from the A-D converter 13. A correlation value calculator 27 uses the multiplied output to calculate the value of correlation between the input signal and the spreading code generated by the code generator 25. A check is made to see if the thus computed correlation value is larger than a predetermined threshold value, thereby specifying the spreading code of the input signal and choosing the cell where the mobile station is staying. The spreading codes that are used to discriminate the cell usually have very long repetition periods. Hence, much time is spent in making a decision about the spreading code of the visited cell by examining the correlation between every long-period spreading code (also referred to simply as a long code) and the input signal.
In the conventional receiver configuration, the receiving fingers Fn.sub.1 to Fn.sub.4 are not equipped with a function of searching for the timing at which the correlation reaches its peak, and since the searcher 15 is provided separately of them to perform the cell search and the multi-path search, no fast searches can be expected. Additionally, a plurality of searchers must be prepared to implement fast searches.
In the CDMA radio communication system communications are performed using the same carrier frequency and the cell identification needs to be made after establishment of synchronization between the spreading code and the received signal. In a system that makes the cell identification on the basis of the kind or phase of the long-period spreading code (IS-95, for instance), the cell search requires a very large number of candidates for the long-period spreading codes. Besides, the repetition period of each long-period spreading code is so long that much time is needed to make a decision about the spreading code.